The long range goal of the project is to understand the role of cellular interactions in the normal development and postembryonic remodelling of, neuromuscular systems. Interactions between motoneurons and their target muscles are a significant factor in the differentiation and maintenance of healthy muscle fibers, and target-derived information regulates the survival and morphology of motoneurons. Activity-dependent interactions are of particular importance during the later stages of development at the neuromuscular junction and during synaptic development within the CNS. Activity levels influence synaptic strength and specificity, and may play a crucial role during the postembryonic remodelling that is a normal part of the maturation of neuromuscular systems. In the proposed experiments, the remodelling of neuromuscular systems during insect metamorphosis will be employed as a model for examining the cellular mechanisms underlying nerve-muscle interactions. During metamorphosis in the moth, Manduca sexta, identified larval motoneurons survive the death of their target muscles and persist to innervate newly-generated muscles of the adult legs. The development of adult muscle is dependent upon the presence of the motoneurons. The availability of precise information about changes in the structure of identified motoneurons and in their expression of voltage-gated ion channels during metamorphosis, makes this an especially attractive model system. The initial aim is to identify the source of muscle precursors and to describe the differentiation of muscle fibers and neuromuscular synapses. The differentiation of voltage- and ligand-gated ion channels of muscle cells will be examined in detail. Experiments performed both in vivo and in vitro will reveal whether it is the proliferation or differentiation of muscle precursors that depends upon the motoneurons, and whether the neural influence is mediated by diffusible factors or requires cell contact. The specificity of the motoneuron requirement will be determined by manipulating identified motoneurons in vivo, and by varying the source and stage of motoneurons in neuron/muscle co-cultures. Further experiments will examine the role of neural activity in regulating the elaboration of motoneuron terminals and the differentiation of muscle cells.